1. Field of Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine or a facsimile machine, for forming color and black/white images based on an image forming process, e.g., electrophotography, electrostatic recording system, ionography, magnetic recording system and the like. More particularly, the invention relates to an electrophotographic image forming apparatus of the tandem type which is provided with a plurality of photoreceptor drums and a plurality of image carrying bodies, and is free from color misregistration after it is commercially supplied and undergoes maintenance and is capable of forming images of high quality for a long period of time. Further, the invention relates to a small color electrophotographic image forming apparatus which is small in size and low in cost, and improved in the operability of an image forming unit.
2. Description of Related Art
Various types of xerographic based image forming apparatus for forming color and black/white pictures, such as printers and copying machines, have been proposed and marketed. In particular recently, much effort has been made to develop color printers with the spread of personal computers, internet, digital cameras, and the like. In the field of image forming apparatus, e.g., color printers, there is a strong demand for developing an apparatus which is capable of forming office-use color images, and is operable at a high speed, compact in size, and low in cost.
To form a color image of high quality by using image forming apparatus, such as a color printer, it is necessary to improve accuracy of color registration, i.e., a registration of color component images. That is, to form a color image, color component images of colors, e.g., cyan, magenta, yellow, and black, are formed on an image bearing member, and those color component images are accurately registered to one another and superimposed one upon another.
An image forming apparatus of the tandem type is known for the image forming apparatus, such as a printer. This type of image forming apparatus includes a plurality of image forming units with photoreceptor drums as image carrying bodies corresponding to colors of toner images to be formed, e.g., cyan, magenta, yellow and black. Those color component toner images are successively formed on the image carrying bodies (e.g., photoreceptor drums) of the image forming units. Those toner images are transferred on a recording sheet or an intermediate transfer member in a superimposing fashion.
The tandem type image forming apparatus includes a plurality (e.g., four) of image forming units for forming color component images. Therefore, to improve the accuracy of color registration of the color component images as formed by the image forming units, it is necessary to locate the color component images formed by the image forming units accurately in predetermined positions. To this end, the image forming units of the tandem type are arranged such that the accuracy of color registration, which is performed when the images formed by the image forming units are superimposed one on another, is improved by controlling the scan start timings of the main scan and the sub-scan by an image exposure unit, which forms an image on the image carrying body by a laser beam, the phase of the polygon mirror for scanning by the laser beam, or the position of the mirror for guiding the laser beam to the image carrying body.
In color electrophotography-based image forming apparatus of the type in which a plurality of image forming units including the image carrying bodies (photoreceptor drums) are provided for each color, it is a common practice that the image forming units may be replaced with new ones for each color, as disclosed in Japanese Utility Model laid-open No. 29148/1987. In some image forming apparatus, a replaceable system is employed in which the respective color image forming units are fixed to a single image forming unit positioning member, and the positioning member is then mounted on the main body, as disclosed in Japanese patent laid-open No. 36346/1996. Either system needs the replacement of several number of image forming units. Accordingly, this fact remarkably increases a mental strain of the user when he or she replaces the an image forming unit with another image forming unit. In a case where the image forming units are assembled into a unit and the support shafts of the image carrying bodies are positioned to the frame of the unit, the following problem arises: an accuracy of positioning the writing unit, the intermediate transfer unit for superimposing the color component images, and the image carrying bodies is deteriorated. In the image forming unit as disclosed in Japanese patent laid-open No. 78686/1998, the position accuracy of the laser-based writing unit and the image carrying bodies, and the position accuracy of the image carrying bodies and the image carrying body transfer drums which are in contact with the image carrying bodies greatly affect the accuracy of color misregistration. A problem of color misregistration arises in the image forming unit disclosed.
In most cases, the color registration of the apparatus main body is adjusted at a factory before it is delivered. When the image forming unit greatly affects the color misregistration, the adjustment equivalent to that at the initial state of the product at the factory is required in a maintenance work, e.g., the user replaces the image forming unit with a new one, after the apparatus is commercially supplied. In this case, to lessen the user""s load, some means to automatically adjust the color misregistration may be used additionally. A color misregistration adjusting technique is present which detects the result of superimposing the color component images by some means, and feeds back the detection result for adjustment. The technique needs a high accuracy detecting mechanism and adjusting mechanism. This fact results in remarkable increase of the apparatus cost. The color misregistration may be adjusted in a manner that the image formed is output, and a quantity of a color misregistration of the image is detected by the eye, and a color misregistration correction quantity based on the detection is input to the apparatus main body. In this approach, the adjusting accuracy is poor, however.
A color misregistration arises from a position accuracy of the image carrying bodies support shafts and a laser beam position accuracy of a ROS. A high position accuracy of several tens xcexcm is required for satisfying a tolerable level. It is actually difficult to satisfy the requirement of such a high position accuracy, and a common practice is to adjust the timings of the laser beams for the purpose of preventing color misregistration. Accordingly, in order to make the user free from those sequential adjusting steps when he replaces the image forming unit including the photoreceptor drum with a new one, it is necessary to exactly position and fix the image carrying bodies support shafts to the apparatus main body.
Before delivery from a factory, color misregistration is adjusted, in both the main scan and sub-scan directions of the ROS, on the basis of the image as finally printed out and according to the writing timings of the ROS and the readout timing clock frequency. The color misregistration depends greatly on the position accuracy of the photoreceptor drums. In spite of this fact, its adjustment is carried out by controlling electrical times, not by adjusting those positions. For this reason, it is necessary to accurately position the image carrying bodies support shafts to the main body.
The conventional image forming apparatus employs a C-path transport system in which a sheet transporting path is curved shaped like a C when viewed from the side of the apparatus. The C-path transport system has the following advantages, and is employed in particular by small desk-top printers.
1) The sheet transport path is short, a time taken from sheet feeding to sheet discharging is short, and the image forming is highly efficient.
2) If the sheet feeding direction is placed in the front surface of the image forming apparatus, the sheet transporting path is opened to the user side. Accordingly, a jam removal operation is easy.
3) The operation of replacing the image forming unit may be performed by opening the front cover or the upper cover. A space occupied by the image forming apparatus when the cover is opened is small.
In the color tandem apparatus containing a plurality of image forming units (photoreceptor drums), it is necessary to exactly position the image forming units (photoreceptor drums) to the main body in consideration of color misregistration or other problems. Further, the image forming units must be constructed so as to allow the user to replace them with other ones. In the C-path transport system, the image forming units are vertically arranged. Accordingly, in the structure where the image forming units are inserted into and removed from the main body in lateral directions parallel to the image carrying bodies support shafts or the unit is inserted into and removed from the main body in lateral directions parallel to the image carrying bodies support shafts as the sheet transporting directions, a space to allow the insertion and removal motions of the image carrying bodies in the image carrying bodies support shaft directions must be provided when taking into account the insertion and removal of the image forming units located in the lower part. Further, a cover opened for jam removal is different from a cover opened for the color tandem machine replacement, so that an advantage of restrictions on the machine installation space is impaired. For the insertion and removal of the image carrying bodies in the sheet transporting direction, an angle of the opening of the cover located on this side must be large, so that the apparatus installation space is large. A manual inserter, transporting rolls and the like are located upstream of a transfer section. If those are disposed at positions where the insertion/removal motion of the image forming unit located in the lower part is not hindered, the transporting distance is long, and the advantages of the C-path transport system is impaired. In Japanese patent laid-open No. 72983/1999, a plurality of image forming units, which are detachably attached to unit receivers formed in the apparatus main body, are inserted and removed horizontally or in lateral directions. Accordingly, the sheet transport path is long, and the advantages of the C-path transport is impaired.
In the image forming apparatus of the tandem type, a very expensive device is required to correct color misregistration of the apparatus after it is commercially supplied. If such an expensive device is not used, color misregistration adjustment will be difficult and its accuracy is extremely deteriorated.
Accordingly, an object of the present invention is to solve the above problems, to accurately position the image carrying bodies to prevent color misregistration, to lessen the mental strain of the user in maintenance work, and to prevent the charging performance and the developing performance of the image carrying bodies from being degraded, and to provide an image forming apparatus of good electric power feeding performance.
Another object of the present invention is to provide a technique to realize a mechanism for the C-path transport system, which is advantageously applied to a small printer, in a color tandem apparatus. In this case, extreme care must be exercised so as not to lower the color misregistration accuracy and not to deteriorate the operability.
Further subjects of the invention are:
(1) To find such an insertion/removal direction as not to hinder the insertion/removal motion of the image forming unit.
(2) To assemble the image forming units into a unit assembly and hence to improve the operability at the time of inserting and removing the image forming units.
(3) To construct such a structure as not to deteriorate the color misregistration.
(4) To reduce the apparatus cost and size by guiding the image carrying bodies support shafts.
(5) To save the widthwise space by commonizing the configuration of the apparatus main body.
(6) To prevent erroneous operations.
The invention is made to solve the problems. In accordance with a first aspect of the present invention, there is provided an image forming apparatus having an image forming unit, in which the image forming unit includes a plurality of image carrying bodies, the image forming unit is a unit assembly, and the unit assembly is formed to be detachable from a main body of the image forming apparatus. The term xe2x80x9cunit assemblyxe2x80x9d means an aggregation of the individual units of the image forming apparatus, which are arranged such that those units exhibit their functions. The unit assembly may be handled as one component.
The plurality of image carrying bodies may be a plurality of latent image carrying bodies or a plurality of intermediate transfer members. The plurality of image carrying bodies may also be a plurality of latent image carrying bodies and one intermediate transfer member. Further, the plurality of image carrying bodies may be one latent image carrying body and one intermediate transfer member or a plurality of latent image carrying bodies and a plurality of intermediate transfer members. In a preferred embodiment, the unit assembly formed to be detachable from the main body of the image forming apparatus is disposed in the main body of the image forming apparatus by downwardly moving the unit assembly. The preferred embodiment involves the following construction: A portion on which the unit assembly is placed is pulled out of the image forming apparatus main body in the substantially horizontal direction, and in this state the unit assembly is put on a unit assembly placement location by downwardly moving the unit assembly, and then the pulled out one is returned to its original position. With such a construction, the portion includes the plurality of image carrying bodies, so that the weight of it is increased. As a result, the insertion or installation of the unit assembly is easy.
The unit assembly may include a handle on an upper surface thereof. This feature makes it easier to install the unit assembly into the image forming apparatus main body. In this case, the unit assembly formed to be detachable from the main body of the image forming apparatus may be disposed in the main body of the image forming apparatus by a single action of downwardly moving the unit assembly. This feature is desirable since it remarkably reduces a mental strain of the user when he replaces the unit assembly with another unit assembly.
A positioning member for positioning the unit assembly when the unit assembly is disposed in the main body of the image forming apparatus may be integrally molded. With this feature, the unit assembly may be accurately positioned, to thereby effectively prevent color misregistration. In that case, the image forming apparatus may further have an exposure unit for forming a latent image on the image carrying body, in which the exposure unit is supported by the positioning member integrally molded. The plurality of image carrying bodies and the exposure unit are supported by the common positioning member. Accordingly, color misregistration may be prevented accurately. In this case, the positioning member and the housing of the exposure unit may be made of the same material. The linear expansion coefficients of the positioning member and housing are equal to each other. The occurrence of the color misregistration, caused by the influence of temperature of an ambience in which the image forming apparatus is placed, may be suppressed effectively. A material of the positioning member and housing is preferably a material formed by mixing glass fiber into polycarbonate, for example.
Preferably, in the image forming apparatus, the image carrying bodies are supported by the image carrying body supporting members, at least one of the supporting members is minutely movably supported by a frame of the unit assembly with a gap, and when the unit assembly is disposed in the main body of the image forming apparatus, the supporting member is positioned and fixed to a housing of the main body of the image forming apparatus. More preferably, in the image forming apparatus, the image carrying bodies are supported by the image carrying body supporting members, at least one of the supporting members is minutely movably supported by a frame of the unit assembly with a gap, and when the unit assembly is disposed in the main body of the image forming apparatus, the positioning member positions and fixes the supporting member. The unit assembly preferably includes a plurality of developing units.
In the image forming apparatus, it is preferable that the developing units are pressed against surfaces of the image carrying bodies to be positioned so that predetermined gaps are present between the developing units and the surfaces of the image carrying bodies.
In the image forming apparatus, the developing units receive drive forces from a developing-unit driving device located in the main body of the image forming apparatus, the developing-unit driving device is different from image carrying body driving device, and in the developing unit, gears are arranged such that a tangential load of the gear, which operates when the developing-unit driving device is driven, does not produce such a component of force as to cancel a pressing force applied to a drum surface of each developing unit.
The unit assembly may include a plurality of charging units. In the image forming apparatus, the charging units are positioned and fixed to shafts of the image carrying bodies, the charging units have mechanisms which are minutely movably supported on a frame of the unit assembly with a gap, and the mechanisms prevent the charging units from turning in a circumferential direction of the image carrying bodies.
The image carrying bodies may be earthed and include ground lines, which interconnect shafts of the image carrying bodies by way of electrically conductive elastic members.
The image forming apparatus may further have a charging device which feeds electric power to a charging unit from outside of the unit assembly by way of a frame of the unit assembly, in which the charging device includes an electrically conductive elastic member which couples a conductive member for feeding electric power to the charging unit located on a frame of the unit assembly and a charging-unit supporting member.
The intermediate transfer members may be cylindrical intermediate transfer drums. The intermediate transfer drums may include a primary transfer drum onto which an image is transferred from the image carrying body and a secondary transfer drum onto which the image is transferred from the primary transfer drum and the secondary transfer drum may be a drum for transferring the image thereon onto a sheet of paper.
Furthermore, the intermediate transfer drums may be supported by intermediate transfer drum shafts, at least one of the intermediate transfer drum shafts may be minutely movably supported by a frame of the unit assembly with a gap, and the shafts of the intermediate transfer drums have mechanisms for positioning and fixing the shafts of the intermediate transfer drums to a housing of a main body of the image forming apparatus when the unit assembly is installed in the main body of the image forming apparatus.
The unit assembly may have four image carrying body drums, two primary intermediate transfer drums, and one secondary intermediate transfer drum. Preferably, the shaft of the secondary intermediate transfer drums is press-fitted into the frame of the unit assembly.
According to another aspect of the present invention, there is provided an image forming apparatus having an image forming unit, in which the image forming unit includes a plurality of image carrying bodies, the image forming unit is a unit assembly in which axial lines of the plurality of image carrying bodies are arranged parallel to one another in a plane containing the axes, the unit assembly is detachable from a main body of the image forming apparatus in a direction of a line perpendicular to the axes in the axes-contained plane.
The image carrying bodies are preferably supported by a single frame. Each of the image carrying bodies may be positioned to the main body of the image forming apparatus. The main body may have first guides for guiding ends of image carrying body support shafts to position the unit assembly to the main body of the image forming apparatus. The first guides may include oblique branch paths for guiding the image carrying body support shafts to fixing positions.
At least one protrusion, which is different from the image carrying body support shafts, may be provided on a frame of the unit assembly and second guides for guiding the protrusion may be provided on a housing of the main body of the image forming apparatus. With this feature, the unit assembly is inserted into or removed from the image forming apparatus main body, while being guided by the first and second guides. Accordingly, the inserting and removing operations of the unit assembly are free from its inclination and twisting.
The image carrying bodies may be photoreceptor drums or may be intermediate transfer drums and photoreceptor drums. In the image forming apparatus of the invention, a sheet feeding portion may be located at a lower position, a sheet discharging portion may be located at an upper position, a direction in which a transfer member is transported at the time of image transferring may be in a range of from 45xc2x0 to 90xc2x0 with respect to a horizontal direction, and the axes-contained plane may be a substantially vertical plane with respect to the horizontal direction. The reason why the transporting direction of the transfer member is set to be in a range of from 45xc2x0 to 90xc2x0 is that an ideal transporting direction angle is as close to the vertical direction as possible, but at least 45xc2x0 or larger is desirable for the transporting direction.